Under your example, why would anyone take that job? People have the ability to vote with their feet. Some other farmer would offer a better job and squash his competition. A labor union isn’t needed to correct that problem.
Are you really that ignorant?
People took jobs like that because there were no other jobs to be taken. Where else would they get work? There were no other farmers. There was no competition.
You know those anti-monopoly laws? They weren’t around when the Unions started.
Not true that another farmer would offer more. Please read (or see the movie) THe Grapes of Wrath. POor people from the dustbowl, who had lost their farms and livelihoods because of natural catastrophe were indigent and desperate.
Their need was exploited by farm owners. You can vote with your feet when you don’t have hungry children and a dying mother.